The invention relates to a honeycomb, particularly as a catalytic converter substrate, with a honeycomb structure comprising a large number of ducts running in the longitudinal direction of the honeycomb, through which a fluid can flow, where the honeycomb possesses foil layers arranged one above the other.
Honeycombs of this type are particularly used as exhaust gas catalytic converters in motor vehicles, although they can also be used in other combustion installations or in chemical engineering, if appropriately dimensioned.
Honeycombs of this kind are known which possess an alternating arrangement of plane and corrugated foils soldered to each other, thus producing fairly small ducts with a cross-section that is essentially sinusoidal or encloses an incircle. In order to achieve the most complete possible catalytic conversion in the honeycomb, the honeycomb must have an appropriate volume to ensure that the retention time of the substances to be converted in the honeycomb is sufficiently long. However, the surface area of the duct walls contained in the honeycomb volume is an essential determinant of the costs, as it causes a corresponding increase in the use of structural material and catalytically active coating material, which usually contains precious metals, such as platinum, palladium and/or rhodium. The increase in substrate mass, as well as in binder and carrier material for the catalytically active precious metals, also increases the thermal inertia of the catalytic converter, as a result of which, for example, the pollutant emission of exhaust gas catalytic converters is elevated during the warm-up phase of vehicle engines. Moreover, an increase in the internal surface area also increases the pressure loss in these and thus the power loss of the drive unit.
The object of the invention is to create a honeycomb that permits highly effective catalytic conversion with low thermal inertia, as well as the inexpensive manufacture of catalytic converters.
According to the invention, the object is solved in that ducts are provided, whose cross-sectional dimension in a first direction is several times the cross-sectional dimension in another direction, particularly a direction perpendicular to the first it has been found that ducts of this kind display a far more effective duct geometry that sinusoidal ducts, for example, in which the gussets or acute-angled inside corners of the ducts have virtually no catalytic effect, despite being coated with catalytically effective material. The efficiency is also improved in comparison with ducts with an approximately circular or isogonal duct cross-section, because the elongated or gap-like duct cross-sections permit better mass transfer transverse to the flow ducts owing to a more favourable ratio of cross-section to wall circumferential area, this being favourable in the case of laminar flow, in particular. Due to the elongated, non-isogonal cross-sections of the ducts, where the duct walls are at very different distances from the centre of the duct, the volume percentage with stagnating or only slow-flowing gas boundary layers, and thus the inhibition of diffusive transport of the pollutants to the catalytically active coatings, is reduced, and the area-specific reactivity, and thus the efficiency of the resultant catalytic converter, is substantially increased. Moreover, savings can be made on substrate and coating material.
The ducts can display a straight, curved or kinked, parallel profile.
The non-isogonal or non-isometric ducts according to the invention preferably account for a volume percentage of the honeycomb that constitutes a non-negligible proportion of the total capacity of the catalytic converter, e.g. more than 5% of the total catalytic converter capacity, particularly preferably almost the total catalytic converter capacity. The honeycomb area designed in accordance with the invention can thus, for example, account for more than 10 percent by volume and preferably more than 25 or 50 percent by volume of the total honeycomb volume.
The areas of the honeycomb designed according to the invention are preferably at a distance from its marginal areas, i.e. its face and/or lateral surfaces. The distance from the marginal areas can be several times the duct height, e.g. more than 5 or 10 times, or a fraction of the honeycomb width, e.g. {fraction (1/20)} to {fraction (1/10)} or more. Advantageously, the entire honeycomb structure is composed of non-isogonal ducts according to the invention, where different areas of the honeycomb can, however, display different duct cross-sections.
In order to improve the efficiency of the catalytic converter, the non-isogonal ducts can extend over a substantial portion of the length of the honeycomb, e.g. more than one-quarter or one-half of the same, preferably over virtually the entire length of the same, where the ducts can be interrupted by areas having different cross-sectional geometries. However, it could be sufficient that the ducts having non-isogonal cross-sections or having a substantially constant heigt over a width of several foil layer distances do extend with their cross-sections as defined above over a length corresponding to several foil layer distances.
The ducts advantageously extend over the entire width of the honeycomb, as a result of which temperature equalisation is not impeded by partition walls and mass transfer is possible over the entire width of the honeycomb, this leading to a more uniform distribution of both velocity and mass over the cross-section of the honeycomb. The retention time of the fluid in the honeycomb, which defines the lower limit of the honeycomb volume, is equalised and increased as a result, thus also increasing the efficiency.
In particular, the ducts can have at least locally or over a portion or preferably over the entire length of the honeycomb a height being at least substantially constant, the ducts extending with this height over a width of the honeycomb of a length corresponding to a multiple (for instance the 2 or 3-to 5-fold or even more) of the average or the greatest height of the ducts or the foil layer distance. Duct walls or duct sections having smaller cross sections can be adjacent these duct sections. Adjacent foil layers therefore extend at least over this width or over a larger width, for instance {fraction (1/10)} or xc2xd or over the entire width of the honeycomb substantially in parallel to each other. Accordingly substantially no or only small cross sections lowerings, for instance being about 25% or less of the duct height, are present over this width.
The ducts can, in particular, be designed in such a way that they extend with at least an approximately equal height over a width corresponding to a multiple (e.g. 3 to 5 times or more) of the mean of maximum duct height. These areas can be followed by duct walls or duct constructions.
In particular, the foil layers have a profile, the profile height of which is small in comparison with the distance between opposite foil layers. This profile can be provided, for example, in the form of punctiform elevations and depressions, these increasing the stiffness of the foil layers and simultaneously improving the adhesion of a ceramic substrate material on the foil layers.
In addition, or as an alternative, the foils can also be provided with a profile, the profile height of which, i.e. the distance between the upper and lower vertex of the profile, is large compared to the distance between the foil layers. In addition to stabilising the dimensions of the foils, this can also influence the flow characteristics of the honeycomb, e.g. in relation to a mass transfer in the transverse direction of the same.
For special applications, e.g. with partial lateral inflow into the ducts, it may be desirable to provide a profile which is of asymmetrical design in relation to a reference plane running through one vertex of the profile, perpendicular to the foil layer. The flow resistance in opposite directions transverse to the profile is different as a result, meaning that the honeycomb acts as a kind of flow diode for the transverse component of the flow in the ducts. To this end, the foil layers can, for example, be folded in zigzag fashion, the adjacent limbs of the fold having different lengths and slopes.
The honeycomb according to the invention can, in particular, be made up of foil layers with identical profiles that are arranged in congruent fashion relative to each other. Honeycombs with curved or bent, gap-like ducts can be constructed particularly easily in this way. Given a suitable profile, however, the foil layers with identical profiles can also be arranged inversely in relation to each other and also display different duct widths in the course of the duct.
Profiling can be achieved in a wide variety of ways, regardless of the profile height. The foil layers can be provided with profiles that extend in the longitudinal direction or transverse to the longitudinal direction of the flow ducts and are designed in the form of ribs or webs. The profiling can extend over the entire length or width of the honeycomb or the width of the flow ducts, or only over part of the same.
In particular, the profile can be designed in such a way that the smallest distance of the foil layers from each other remains virtually unchanged or is not significantly reduced, so that the flow resistance through the honeycomb is not increased. The profile can, in particular, be realised by folding the foils, so that the honeycombs are easily manufactured while displaying great stability. The slope of the profiled areas in the longitudinal direction of the ducts, in particular, can be small compared to the distance between foils, this keeping the pressure losses low.
The honeycomb structure can, in particular, be designed in such a way that the Nusselt number, referred for comparison to a specific mass flow, such as is typical for automotive applications, for example, is  greater than 4.5, preferably  greater than 6 for an area of the honeycomb of  greater than 10 percent by volume, preferably  greater than 25%, particularly preferably  greater than 50%. In these relations, the values given refer to a diffusion distance of 0.5 mm, which corresponds to the radius in the case of flow ducts with circular cross-section, for example. In particular, Nusselt numbers of 15 can readily be achieved with ducts of large cross-section with carrier profiles for catalytically active material running parallel to the direction of flow and permitting flow around their full circumference, which can have edges around which flow is possible. In particular, the honeycomb can be designed in such a way that a mean Nusselt number of  greater than 4.5, preferably  greater than 6 results for it. For comparison, it can be mentioned that the Nusselt number for gap-like ducts extending over the width of the honeycomb is approximately 8.
The cross-sections of the ducts designed according to the invention can be arranged in such a way that they extend over cross-sectional areas of the honeycomb in which temperature differences of more than 10xc2x0 C., preferably more than 50xc2x0 C. exist during the start-up phase of catalytic converter operation. In particular, starting from the outer sides of the honeycomb, which are the coldest during the start-up phase, the ducts can extend over 25%, preferably over half of the honeycomb cross-section towards the centre axis or plane of the same. Catalytic converter carrier elements with edges around which flow is possible, particularly carrier elements permitting flow around their full circumference, can be provided in these areas.
Owing to the great width of the flow ducts, the catalyst coating can be thicker than in conventional honeycombs. For instance, with a foil thickness of approximately {fraction (5/100)} mm, the coating thickness can be 5 to {fraction (25/100)} mm or more, corresponding to a ratio of coating thickness to foil thickness of 1 to 5 or more. The ratio can also be  greater than 10 for special applications. This substantially reduces the sensitivity of the catalyst to catalyst poisons.
In order to increase the dimensional stability of the ducts and the resistance of the honeycomb to thermal shocks, the foil layers can be provided with stiffening elements, the advantageous design of which is described further below and which are preferably provided in or adjacent to the areas of the honeycomb having duct sections with non-isogonal cross-sections. The stiffening elements can be integrated in the foil layers, e.g. in the form of profiles extending in and/or transverse to the longitudinal direction of the flow ducts, and can, for example, be designed as meandering foil layer folds with contacting fold webs. It is also possible to insert additional, separate stiffening elements into the honeycomb which, for example, support relatively large duct wall areas and can act on the duct wall surfaces in punctiform or linear fashion for this purpose. The stiffening elements, e.g. in the form of wire pins, can be offset relative to each other in the supporting areas formed by them, which can be designed as planes, in such a way that they result in little or virtually no constriction of the duct cross-sections and generate no major vortex zones when the fluid flows around them. The stiffening elements can also be interconnected. Regardless of the design of the stiffening elements, they can be distributed in the honeycomb in organised fashion, e.g. in preferred directions or planes of the honeycomb or in symmetrical patterns, or they can also be distributed randomly. Honeycomb areas with accumulations of stiffening elements can alternate with areas with few or no stiffening-elements, this making it possible to produce zones with different mechanical properties, such as elevated stiffness. The zones can be located within the honeycomb or in its peripheral areas and can alternate in the axial and/or radial direction with unstiffened areas of high extensibility. In this context, the dimensions of the stiffening zones transverse to the flow ducts is advantageously a multiple of the distance between foils. To obtain a zoned structure of the honeycomb, the stiffening elements integrated in the foil layers can similarly also display zones of different stiffness or have an appropriately dimensioned length. The stiffening elements can be provided in the interior of the honeycomb and/or in the area of the face and lateral surfaces of the honeycomb. The stiffening elements are preferably arranged in each case such that they cover none, or only small surface areas, of the duct walls.
The stiffening elements can be arranged in such a way that relatively large self-supporting foil areas result as duct walls, where the distance between supports, i.e. the distance between the points of contact of the stiffening elements on the foil layers, is a multiple of the shorter cross-sectional dimension of the ducts or up to the maximum distance at which sufficient stability is still ensured under the operating conditions of the honeycomb. The points of support can be arranged in punctiform or linear fashion or in planes.
The stiffening elements, which can be arranged in aligned supporting planes or lines, as well as at spatially offset, non-aligned points, are preferably arranged in such a way that a fluid can flow around them without any reduction of the cross-section.
In order to stiffen the honeycomb structure resulting from the build-up of the foil layers, e.g. from the laying of foil strips in meandering fashion, isolated, one-dimensional joints can be provided which connect the individual foil layers, which can be profiled or unprofiled, to each other or to further stiffening elements and which are specifically inserted into the honeycomb for this purpose. The joints can, at least in certain directions in space, particularly display greater strength and load-bearing capacity than soldered joints and, in particular, be produced by positive, non-positive and/or material connections, such as by notched tabs or webs which can be fixed in adjacent foil folds, at least in one direction, or fed through apertures, by beading or twisting cut foil sections or, for example, by spot welding methods. The joints can be arranged in preferred directions or planes of the honeycomb or in symmetrical patterns, but they can also be distributed randomly. In given directions, the joints can be provided at every point of contact between adjacent foil layers, or only at every second, third point of contact, etc. The described stiffening of the honeycomb by means of isolated joints can be applied not only to honeycombs with non-isogonal duct cross-sections, but also to those with conventional, isometric (e.g. hexagonal or sinusoidal) duct cross-sections.
The honeycomb is advantageously provided with stiffening elements which run transverse to the flow ducts, this preventing excessive changes in the distance between foil layers being brought about by bending induced by temperature fluctuations. In this context, the stiffening elements can, for example, be designed as wires, strips or layers of expanded metal, or as folds in the foil layers. The stiffening elements can support the foil layers or penetrate them, in which case they can be connected to the foil layers in a manner capable of withstanding tensile forces or passed through them loosely.
Given a suitable profile height of the foil layers, the stiffening elements oriented parallel to the foil layers can also act on several foil layers arranged one above the other.
As an alternative, or in addition, stiffening elements can be provided which are arranged perpendicular to the foil layers and penetrate several foil layers arranged one above the other. These stiffening elements can, for example, have the form of wires or strips, or of wall areas made up of foil folds.
If the foil layers are profiled, additional stiffening elements are advantageously provided that are a long distance apart, e.g. a multiple of the duct height, and that can be of separate design. They are particularly located at those points where the forces acting on the foil layers are more intense, such as in the inlet and/or outlet areas. The additional stiffening elements can be connected to each other and/or to the housing in force-dissipating fashion and, in this context, form planes or lines of increased stiffness, which can be some distance apart in the longitudinal direction of the honeycomb. The stiffening elements can alternatively or additionally also be located in the foil profiles and optionally fastened to these, e.g. in foil profiles in the form of web-like layer folds. The stiffening elements can also be provided in the area of outside and/or partition walls or partial walls of the honeycomb and, in particular, connected to these in force-dissipating fashion. The stiffening elements can be of one-dimensional design in each case, e.g. in the form of wires, pins, strips or foil folds, but they can also be designed as extended-metal layers or the like, or also as joints between foil layers or sections.
In this context, the stiffening elements can be fastened to the foil layers by non-positive, positive or material connections, e.g. trapped in foil folds, which can also be formed by notched tabs, or twisted in order to form a positive connection. Adequate fastening may, under certain circumstances, already be obtained by coating the prefabricated honeycomb, e.g. with the ceramic compound that carries the catalyst.
Advantageously, the width of the respective stiffening elements is small compared to the length of the flow ducts, so that the catalytically active duct walls are only marginally covered by the stiffening elements and the flow characteristics of the honeycomb remain virtually unaffected. Thus, the necessary width of the stiffening elements, which in each case depends on the given profiling of the foil layers, among other things, can be ⅕ or {fraction (1/10)} to {fraction (1/100)} of the length of the flow ducts, without being limited to these values.
In each case, the stiffening elements preferably extend over a length corresponding to several times the distance between foil layers, particularly preferably over the full extent of the cross-section of the honeycomb.
In order to stabilise the foil layers and enlarge the area-specific conversion coefficient on the catalytically active surface, the foil layers can be provided with longitudinal ribs which run parallel to the flow ducts and extend over a part or the whole of the length of the flow ducts. In this context, the height of the longitudinal ribs is preferably small compared to the height of the ducts, e.g. equal to half the duct height or less, so that the local increase in turbulence is not cancelled out again by an increase in thicker boundary layers.
Foil folds with a height greater than the distance between foil layers can also be provided for stiffening, the individual fold webs being fastened to each other. To this end, the fold webs can be directly fastened to each other in non-positive and/or positive fashion; other stiffening elements can also serve this purpose, e.g. in the form of wires or strips running perpendicular and/or parallel to the foil layers.
In order to stiffen the honeycomb structure released or disengaged foil layer sections can be provided supporting adjacent foil layers.
All in all, regardless of the specific configuration, different stiffening elements can be fastened to each other, as a result of which two or three-dimensional systems of stiffening elements can arise.
The stiffening elements can, in particular, be connected to the housing or to partition or outside walls of the honeycomb in tension-absorbing fashion. In this context, the partition and outside walls can be of rigid or elastically deformable design, e.g. in the form of fold webs of the foil layers connected to each other.
Furthermore, the foil layers can be provided with notched tabs which serve to support the foil layers against each other and to permit gas exchange between adjacent ducts or to enlarge the surface area of the catalyst. The notched tabs are advantageously designed in the form of webs running in the longitudinal direction of the ducts.
According to another configuration, flow deflectors can be provided in the region of the flow ducts. e.g. within or at the face ends of the same, due to which flowing fluid volumes automatically continue with a lateral and/or vertical offset, which can also be advantageous with honeycombs not designed according to the invention, e.g. with isometric duct cross-sections. As a result, the length of the flow paths can exceed the length of the honeycomb, for instance by 5 to 20% or more. The offset in each case can, for example, be one-quarter to one duct height, i.e. the extent of the smaller cross-sectional dimension, or more. In particular, the flow deflectors can be designed in such a way that they automatically mix the fluid passing through one duct section with partial volumes of the fluid passing through adjacent duct sections, with which there was little or no mass transfer. These adjacent duct sections can be separated by constrictions, areas of lateral deflection, such as foil layer folds, duct walls or as a result of a large duct width which exceeds the lateral diffusion path in the retention time of the fluid in the duct section, with the result that diffusive mass transfer virtually no longer takes place. The flow deflectors can, for example, be realised by an appropriate design of the duct walls, e.g. with areas running at an angle to the direction of flow, by additional deflection devices, such as baffle plates with deflecting bevels at an angle to the direction of flow in and/or at the face ends of the ducts, and/or by laterally offsetting consecutive sections of the honeycomb with partition walls which divide media volumes guided upstream in a common flow duct as a result of the offset of the partition walls and combine separated volumes in a flow duct upstream. The deflecting bevels, regardless of their design, advantageously lie flat and at an acute angle of less than 45 to 30xc2x0, preferably less than 10xc2x0, to the direction of flow, and can also be of curved design. The flow deflectors are advantageously designed in such a way that they essentially do not reduce the size of the duct cross-sections over a section of the honeycomb and thus do not increase the flow resistance of the honeycomb at all, or only slightly. To this end, the offset of one fluid volume can be coupled with the offset of an adjacent volume element in another direction, e.g. the opposite direction, where the overall cross-section can remain approximately unchanged over the length of the deflection area. To this end, the duct cross sections can, for example, simultaneously vary in both height and width over the length of the duct while retaining a constant cross-sectional size. At the position of the deflectors, the flow of ducts or duct areas adjacent to each other in the direction of offset of the fluid volumes can be coupled, thereby permitting an exchange of fluid. The deflectors can, in particular, act as flow dividers, e.g. by means of adjacent bevels with opposite inclinations, so that a volume flow is divided and deflected in different directions.
At the inlet and/or outlet areas of the honeycomb, the foil layers can display areas which are inclined relative to the principal plane of the foil layers. These areas can, for example, be integrally moulded on the foil layers or on profiles that are to be inserted into the honeycombs at the face end and permit flow around or through themselves. The scoop-like effect of these areas improves the inflow behaviour in the event of inclined inflow into the honeycomb. Furthermore, the inlet or outlet area of the honeycomb can be stiffened by means of additional stiffening elements or foil layer folds, this avoiding pressure losses resulting from undesirable movements or non-uniform changes in the distance between foil layers. In particular, this also applies to the inlet and outlet areas of scoop-like design.
In the event of angled flow of the fluid in relation to the longitudinal axis of the honeycomb, it has proven particularly favourable to design the inlet and/or outlet area of the honeycomb according to the invention in such a way that the inflow direction is parallel, or at an angle of less than 90xc2x0, to the plane of the foil layers. This particularly applies to gap-like ducts that extend over a major part of the width, or over the entire width, of the honeycomb.
According to a particularly advantageous configuration, the sides of the ducts are open towards the outside of the honeycomb in the inlet area of the honeycomb, so that the fluid can flow into the space between the foil layers in a direction transverse to the longitudinal direction of the flow ducts over part of the length of the ducts. This preceding enlarged inlet area around the face end of the honeycomb, which can be realised, for example, by leaving a lateral gap between the inlet pipe and the foil layers, makes it possible to set a more uniform, and thus lower, flow velocity in the inlet area of the honeycomb, which leads to a longer retention time of the fluid here and to less stress from mechanical vibrations, while also reducing the thermal stress on the honeycomb by distributing it over a larger area. As a result, particularly effective catalytic use is made of the inlet area of the honeycomb, the volume of the catalytic converter can be reduced and the honeycomb can be charged with hotter media with less risk of hot-spots. As a result, the catalytic converter can, for example, be installed closer to the engine in motor vehicles, making it possible to shorten the cold-start phase with low catalytic converter activity. The outer edge of the face-end plane and of the honeycomb is heated more intensively, more uniformly and deeper into the honeycomb as a result, thus substantially increasing the efficiency as a whole. This design can also be realised regardless of whether the honeycomb structure has ducts according to the invention.
It follows from the above that it is also easily possible for several honeycombs to be arranged one behind the other and for these to display different duct geometries or to be rotated about their longitudinal axes relative to each other, this permitting further homogenisation of the medium flowing through the honeycomb. Moreover, it is also possible for honeycomb areas designed according to the invention to alternate, in the longitudinal and/or transverse direction of a honeycomb, with others of a different design, e.g. with a different, possibly conventional, duct structure or with an open substrate structure permitting virtually unobstructed fluid exchange transverse to the direction of .flow over substantial areas of the substrate. The duct walls, which prevent fluid exchange transverse to the direction of flow and separate partial fluid flows from each other, can also be provided with apertures, through which a lateral fluid exchange is possible and which can extend over the greatest possible length of the honeycomb, e.g. over more than 15 or 25%, up to virtually the entire length of the honeycomb.
A particularly advantageous design of a honeycomb is one where several mixing zones (including the inlet and outlet zones), where mixing primarily takes place, alternate with several reaction zones, where the reaction primarily takes place. The honeycomb thus preferably displays at least two or more, for example ten or more, reaction zones separated by mixing zones. In this context, the honeycomb can also be assembled from several individual elements, each of which engages the front ends of an adjacent honeycomb, thus forming a continuous reaction and flow chamber. The ratio of the sum of the lengths of the reaction zones to the sum of the lengths of the mixing zones is  greater than 2, preferably 5 to 20 or more. In this context, the length of the comparatively short mixing zones can be 2 to 20 times the gap width or height of the flow ducts. The mixing zone and the inlet zone are characterised in that vortices are produced in them and-that the flow resistance is thus essentially determined by the form resistance and extensive deflection of the flow paths exists as a result of obstacles to flow. To this end, the flow cross-section can, for example, be provided with profiles running at an angle to the direction of flow, such as webs of expanded-metal layers, inlet profiles, wires or the like, the surfaces of which against the medium flows running at an angle of  greater than 15xc2x0, preferably 45 to 90xc2x0, relative to the direction of flow. However, both with angled inflow into the honeycomb and with inflow in the longitudinal direction of the same, a high form resistance is also generated by the duct structure of the honeycomb in the inlet area owing to the abrupt transition from turbulent to laminar flow and the vortices developing as a result, without having to provide flow-deflecting baffle plates or the like for this purpose. In contrast, the reaction zones are characterised by a high frictional force component in the flow resistance, the result being that micro-turbulence zones are present here. In these zones, the carrier elements preferably run parallel to the longitudinal direction of flow, or at an angle of up to approximately 10xc2x0.
Taken as a whole, the ratio of the form resistances of the mixing zone to the frictional resistances of the reaction zone can be 2.5 or more, i.e. the pressure loss in the mixing zone referred to a unit length is 2.5 or more times the pressure loss under the flow conditions prevailing in the reaction zone. As a result, a honeycomb with several zones having markedly different functions is created, where strong transverse mixing takes place in the mixing zones as a result of accelerated flows and vortex formation and where more micro-vortices or shear vortices occur in the reaction zones as a result of static friction.